The present invention relates to the communication of digital data using trellis coded modulation, and more particularly to a method and apparatus for incorporating a rotationally invariant trellis encoding/decoding scheme into a quadrature phase shift keyed (QPSK) or quadrature amplitude modulation (QAM) transmission system. One of the various applications for which the present invention is particularly well suited is in the transmission of digital television signals.
Digital data, for example, digitized, compressed television (NTSC) or high-definition television (HDTV) signals can be transmitted over terrestrial very high frequency (VHF), ultra-high-frequency (UHF), satellite channels or cable television analog channels to end users. In order to communicate digital data via an analog channel, the data is modulated using, for example, a form of pulse amplitude modulation (PAM). Typically, quadrature amplitude modulation (QAM) or single-sideband (SSB) modulation is chosen to efficiently use the available channel bandwidth. QAM is a quadrature, or orthogonal combination of two PAM signals. When viewed as coordinates of a plane, the combined PAM signals form a signal space or "constellation" of possible transmission levels. Each transmitted constellation point is called a symbol. For example, two independent, quadrature four-level AM signals form a 16-QAM constellation which encodes four bits. A 32-point constellation can be formed with dependent six-level AM quadrature signals, encoding five bits per symbol. In systems that have a lower carrier-to-noise ratio (CNR) than can be tolerated by QAM, lower modulation orders are useful, such as QPSK having a four-point constellation.
In pulse amplitude modulation, each signal is a pulse whose amplitude level is selected from a fixed set of levels. In 16-QAM, each of the quadrature PAM signals select from uniformly spaced, bipolar amplitudes scaled from amplitude levels -3, -1, 1, 3. Spectral efficiency in digital communication systems is defined as the number of transmitted information bits per second per unit of bandwidth, i.e., the ratio of the data rate to the bandwidth. Modulation systems with very high bandwidth efficiency are employed in applications that require high data throughput with small available bandwidth. QAM and SSB provide bandwidth efficient modulation, which can provide very low bit error rates when used with high efficiency forward error correction codes such as trellis coded modulation (TCM).
Trellis coded modulation has evolved as a combined coding and modulation technique for digital transmission over bandlimited channels. Unlike the traditional application of convolutional codes to two-level PAM, which increases the bandwidth used in transmission, TCM increases the constellation size instead. In TCM schemes, a sequence of "coded" bits are convolutionally encoded into a sequence of groups which partition the symbol constellation. For each encoded group of a QAM constellation, a number of "uncoded" bits are transmitted by selecting a unique constellation element of the group. Most TCM schemes map one step of the convolutional code trellis to one transmission symbol which consists of two QAM components (I, Q). Such two-dimensional (2-D) codes achieve a throughput of an integer number of information bits per 2-D symbol.
At a receiver, the sequence of transmitted groups is decoded by a soft-decision maximum likelihood (ML) convolutional code decoder. Such TCM schemes can improve the robustness of digital transmission against additive noise by three to six dB or more, compared to uncoded modulation at the same information rate. One widely used technique for efficient ML decoding of convolutional codes is the Viterbi algorithm disclosed in A. J. Viterbi and J. K. Omura, Principles of Digital Communications and Coding, New York, N.Y., McGraw Hill 1979. It is known that decoding of high-rate R convolutional codes can be simplified by using "punctured" codes, which are obtained by periodically deleting some of the output bits of a lower rate code. A rate 1/n code can be punctured to a rate m/k and can be easily decoded with simple modifications to a rate 1/n decoder. An example of such a decoder is provided in commonly assigned, copending U.S. patent application Ser. No. 08/054,642 filed on May 5, 1993 for "Apparatus and Method for Communicating Digital Data Using Trellis Coding with Punctured Convolutional Codes."
Fast recovery from phase ambiguities is very important for robust modem design. Of all the tracking loops in a typical receiver, such as the automatic gain control, adaptive equalizer, and carrier timing loop, the carrier recovery loop is often the most fragile, resulting in noise. Phase ambiguities can cause a carrier timing slip, requiring a major resynchronization of the forward error correction (FEC), leading to a burst of errors at the FEC output. The Viterbi algorithm (or other sequence estimator used) must detect the event and restart the decoding. Therefore, it would be desirable to provide a coding method that quickly recovers from a phase rotation without causing the FEC to change state. Such a coding method would be particularly useful in the design of a receiver that can cancel large amounts of phase noise introduced in the mixing process.
To robustly track phase jitter, the carrier timing loop bandwidth is typically opened, causing the signal-to-noise ratio (SNR) in the loop to degrade. This leads to exposure to phase flips, limiting the ability of the receiver to handle phase noise. Quick recovery from carrier timing loop slips enables a more aggressive phase noise cancellation to be implemented without the risk of a large error burst appearing at the output of the FEC.
One problem that has been encountered with multilevel modulation techniques, particularly when used with trellis coding, is that 90.degree. phase ambiguities may occur in the signal received from a communication channel. Such phase ambiguities make it difficult to determine the absolute phase of the symbol that has been received. Decoding errors will occur when incorrect assumptions are made as to whether one point or another point in the same group, but offset by 90.degree., has been received.
It would be advantageous to provide a rotationally invariant trellis encoding/decoding scheme for use in a QPSK or QAM transmission system or the like. Such a scheme should resolve all multiples of 90.degree. phase ambiguities. Quick recovery from phase flip in a receiver should be provided. Any propagation of errors should be insignificant, and coding gain should not be adversely affected.
The present invention provides a rotationally invariant trellis encoding/decoding scheme enjoying the aforementioned advantages.